The invention relates to a filter for continuous filtration of a suspension under pressure, with a pressure vessel, rotating filter elements disposed in the pressure vessel, where a filter element passes through one filter cycle for each rotation, with filter zones in liquid and gaseous phase, also with filtrate channels that are connected between the filter elements and separate filtrate outlets for each filter zone, as well as a device to control the individual filtrate zones by means of a control disc, where the control disc has filtrate outlets for liquid and gaseous phase.
Filters of the kind mentioned above are used as pressure filters to dewater fibrous suspensions, among other materials, but also more and more to dewater suspensions containing mineral solids, such as coal or ores, in order to achieve a higher degree of dewatering, i.e., a higher dryness. One example of a pressure filter for fibrous suspensions is described in DE 36 14 668 A1. Here, a pressure disc filter is shown, however it would also be possible in principle to use a pressure drum filter. A filtration cycle usually consists of a so-called cake formation, i.e. formation of a filter cake, or a filter mat in the case of fibrous suspensions, on the filter element. The following refers generally to filter cakes, however it also relates in the same way to a filter mat made of fibrous pulps. This cake formation can also take place in several stages, as is described in DE 36 14 668 A1. Another variant of a pressure filter is described in EP 0 596 857 A1. When the filter cake has formed, air is blown through it. In vacuum filters, the liquid contained in the filter cake is removed by suction, and in pressure filters it is pressed through it as a result of the higher pressure. Then filter cake washing may be included as the next stage in the filtration cycle, and the washing liquid here can then also be removed again from the filter cake by the pressure differential prevailing on the filter element (vacuum or internal vessel pressure). After this, the filter cake is removed from the filter element, which can be achieved by means of water or air jets (fibrous mat), a compressed air snap blow, or doctors. Then a new filtration cycle begins. At the beginning of the filtration cycle, it is important to guarantee that the filter cell and the filtrate pipe, to which several filter cells may also be mounted, are vented. In pressure filtration, the filter cell(s) and filtrate pipe volume are also under the working pressure in the vessel. As a result, venting takes place abruptly at the beginning of the cake formation zone due to the drop in pressure to the atmospheric pressure prevailing outside the vessel and results in considerable pressure shocks, swirling and turbulence in the cake formation zone. When this happens, the expanding volume of air is usually much larger than the filtrate quantity to be removed in the cake formation zone.